Air moving propellers and turbines have been provided with slots through the blades thereof or assembled with overlapping blades in close proximity. These slots may be formed as scoops to enhance rather than disrupt the flow on the concave or suction side of the propeller or turbine blades to prevent flow separation (sometimes called cavitation). Such propellers or turbines are not used in gas-to-liquid or liquid-to-liquid mass transfer applications. The flow patterns introduced by the slots or gaps in impeller blades provided by the invention are effective to break up bubbles which tend to grow due to the coalescing of the gas or liquid being dispersed on the suction side of the blades thereby enhancing the efficiency of mass transfer and the mass transfer coefficient kLa of the mass transfer process. Propellers, turbines and blades with slots designed to prevent flow separation on the suction side of the blades and multi-blade designs are shown, for example in the following patents: Faber, U.S. Pat. No. 2,003,073, May 28, 1935; Chajmik, U.S. Pat. No. 3,044,559, Jul. 17, 1962; Sheets, U.S. Pat. No. 3,195,807, Jul. 20, 1965; Schaw, U.S. Pat. No. 4,102,600, Jul. 25, 1978; Levin, et al., U.S. Pat. No. 4,130,381, Dec. 19, 1978; Thompson, U.S. Pat. No. 4,285,637, Aug. 25, 1981; Zeides, U.S. Pat. No. 4,636,143, Jan. 13, 1987; Spranger, U.S. Pat. No. 4,913,670, Apr. 3, 1990; Schindling, DE 182,680, Mar. 26, 1907; and a slotted scimitar shaped blade known as the Velmix which has curved slots spaced inwardly from the tips of the blades.